In a typical radio communications system, radio communications terminals referred to as radio terminals or user equipment terminals UEs communicate via a radio access network (RAN) with other networks like the Internet. The radio access network (RAN) covers a geographical area which is divided into cells, with each cell being served by a base station, e.g., a radio base station (RBS), which in some networks is also called a “NodeB” or an enhanced Node B “eNodeB.” Each base station typically serves several cells. One common deployment is 3-cell base station installations, where a base station serves three cells. A radio terminal is primarily served by a serving base station in a serving cell in which the radio terminal resides. In some technologies, communication links are not only established between a particular radio terminal and its serving cell, but also between the radio terminal and other cells. In this case, the terminal is served by multiple base stations using what is referred to as macro diversity or soft handover.
A base station sends signals to and receives signals from radio terminals. The signals may either be dedicated signals to and from specific radio terminals, multicast signals intended for a subset of the radio terminals in a cell, or broadcast signals from the base station to all radio terminals in a cell. A base station broadcasts information to all the radio terminals in a cell using the broadcast channel of the serving cell.
Small scale base stations have recently been introduced that are connected to broadband internet service and provide coverage for very small areas sometimes called femto cells. Femto cells are similar to WiFi “hotspots” but are part of a cellular network rather than a wireless local area network (WLAN). The femto base stations work in many ways like a larger “macro” base station would, but on a much smaller scale with low output power designed for small spaces such as apartments, houses, offices, etc. Pico is another name for these small base stations meaning “small,” and in this case, “femto” means even smaller. Femto base stations provide a better signal in smaller interior or closed spaces where signal quality between regular macro base stations and mobile phones is poor due to the proximity of macro base station towers or just due to the material of the building or other obstructions blocking the signal. Instead of using a traditional base station, the mobile terminal uses the femto base station to gain access to the IP network.
FIG. 1 shows an example of a cellular communications system that includes a small scale base station and a traditional macro base station. A first building 1 includes a radio terminal 2 that receives radio signals from a macro base station 3. The macro base station 3 is coupled to a core network 5 either directly or through a radio access network 4. The core network 5 provides access to the Internet 6 and other networks. A second building 7 includes another radio terminal 8 that receives radio signals from a small scale base station 9. The small scale base station 9 may be coupled, typically via some broadband access mechanism (wired or wireless), to the core network 5 either directly or through a radio access network 4. Again, the core network 5 provides access to the Internet 6 and other networks. Because the small scale base station is located inside the building 7 and is typically only intended to provide coverage within and in close proximity to the building 7, its transmit power can be considerably lower than that of the macro base station 3, which has a much larger and varied coverage area, while still providing high data rate service.
Small scale base stations usually serve small cell areas that have a restricted group of users such that only users in that group can receive service from the small scale base station. Often, a customer may buy a small scale base station in a retail store and install it by connecting it to a power outlet and the data network in a home, office, school, etc. These small scale base stations are sometimes called home base stations, femto base stations, pico base stations, etc. A larger scale macro base station may also only allow access and provides service to a restricted group of users, but a small scale base station is the more typical example. For this application, any base station that only allows access and provide service to a restricted group of users is referred to as a closed subscriber group (CSG) base station. A CSG base station serves one or several CSG cells which only permit access to a restricted set of radio terminals. A CSG cell may also be used to compile networks with restricted access, e.g., to support corporate networks. A base station that is not so closed or restricted in this way is referred to as an “open” base station.
Current cellular radio systems include for example Third Generation (3G) Universal Mobile Telecommunications System (UMTS) operating using Wideband Code Division Multiple Access (WCDMA) and Fourth generation (4G) systems, like the Long Term Evolution (LTE) of UMTS operating using Orthogonal Frequency Division Multiple Access (OFDMA). One important focus area in the LTE and System Architecture Evolution (SAE) standardization work is to ensure that the new network is simple to deploy and cost efficient to operate. The vision is that the new system will be self-optimizing and self-configuring in as many aspects as possible. One such aspect is automatic incorporation and handling of closed subscriber group cells (CSG cells) on the same frequency band as the traditional macro cell layer with essentially no configuration.
A CSG base station installation may be motivated by various expectations and presents various issues. For example, an end-user connected to his/her CSG base station hopes to achieve a higher price/performance ratio as compared to the macro network while still receiving the same services as when connected to a macro base station. The user may also expect higher data rates and better service quality than when camping on a macro cell. Another expectation is easy and essentially automatic installation procedure of the CSG base station which includes support of handovers to/from the CSG base station for authorized users without extensive manual configuration. One issue arises from a CSG base station possibly having lower call management capabilities than a macro base station, and therefore, the CSG base station may be less proficient at performing admission and authorization functions. If so, many non-authorized handover requests to the CSG base station, and subsequent handover rejections in response thereto, will hamper the performance of the CSG base station. Another issue is the desirability of convenient addition of new users who are authorized to use the CSG base station. Furthermore, it is beneficial if operators can sell a CSG base station where all users with a particular subscription with the same operator are authorized to use any, or a subset, of the CSG base stations.
Handovers in LTE and WCDMA are mobile-assisted, which means that the radio terminal reports to its serving base station measurements the radio terminal has made of the quality (e.g., signal strength) of signals received from the serving base station and from other cell alternatives associated with handover “candidate” base stations. The other cell alternatives are identified using cell identifiers. Non-limiting examples of such physical identifiers are the downlink scrambling code number of the cell in WCDMA and the physical cell identifier in LTE describing a reference signal waveform of the cell. The radio terminal may either report another cell also served by the same base station (assuming the base station serves multiple cells) or another cell served by a different base station.
A neighbor cell relation (NCR) list is a list associated with a cell listing relevant (e.g., neighboring) candidate cells for handover. For each candidate cell, the list information includes: cell identifiers including both physical cell identities and globally-unique cell identities, connectivity information, e.g., how a communication link can be established between the serving base station and the candidate cell's base station, and cell type information, e.g., CSG cell, macro cell, micro cell, etc. The neighbor cell relation list may be stored in the base station, but it may also be stored in other nodes, possibly with regular updating of the NCR lists in multiple nodes.
A radio terminal's measurement report transmission is typically triggered, i.e., event-triggered, event-triggered periodic, or periodic. For event-triggered reporting, the radio terminal sends a report to its serving base station when a configured criterion is met. One example of such a criterion is that the quality of a new cell is measured during a predefined time within a range between the serving cell and a predetermined offset. The predetermined time and offset are provided by the serving base station to the radio terminal. Another example of such a criterion is that a cell that previously was within a range between the serving cell and a predefined offset moves outside the range for a predefined time. Event-triggered reporting results in one measurement report upon triggering of an event. For event-triggered periodic reporting, the radio terminal continues to send measurement reports periodically after triggering of an event—either for a predefined number of periods, or until a different triggering condition is met. For periodic reporting, the radio terminal regularly reports measurements. This was typically the case for second generation systems such as GSM.
The radio terminal typically considers all cells when investigating whether a triggering event is met. On the other hand, it is preferred that the radio terminal does not consider and report alternative cells that are somehow indicated as forbidden for access. One way to communicate the forbidden status of multiple cells is for the serving base station to broadcast a black-list of cells on the serving cell's broadcast channel (i.e., the serving cell broadcasts a black list of cell identifiers). Another way is for the radio terminal to only report cells included in a “white” list broadcast by the serving base station on the serving cell's broadcast channel. The white list can be seen as an inverse black list. If there are many cells on the black list, it may be more efficient to signal the white list or vice versa. A third way is for each base station serving a prohibited cell to broadcast a message over its broadcast channel that indicates that the prohibited cell is forbidden for access. All radio terminals should check this indication before reporting a cell. Examples of such indicators are “Cell Barred,” “Cell Restricted,” or “CSG Cell.”
If a CSG base station is deployed as a typical “open” base station, then the CSG base station will likely process many handover requests for non-authorized radio terminals that are not part of the CSG, and as a result, must also reject these requests. This generates a significant and unnecessary processing load for the CSG base station. So there are a variety of problems with handover-related signaling and CSG cells.
But there are also problems with radio terminals not reporting handover measurements for CSG cells. One is how to initialize a newly-deployed CSG cell since it may not be considered as an allowed candidate cell by the radio terminal. Another problem is that a CSG cell's base station may be turned-off during long periods of time, especially if the CSG base station is used in a private home. The network's status may have changed since that CSG cell was active, e.g., the local base station identifiers may have been reallocated or modified, which means that neighbor cell relation information has changed. Also, a CSG cell may have been moved since it was powered on the last time. For example, a CSG cell may have been moved to an office, another business site like a conference facility or client site, or another home. The previous neighbor cell relation information for such a CSG cell may be of little value because the CSG cell's prior neighbors are likely no longer neighbors.